Tear seams for air bag door covers are currently produced by a number of different methods. One technique is to mold a thinned section into the cover during its manufacture. This method provides a designed-in, relatively weaker tear seam region along which the cover separates in response to air bag deployment. However, molded-in tear seams of this type have a tendency, over time, to show-through as a visible depression on the outer class A surface of the cover. Vehicle occupants can, therefore, see the pattern of the tear seam in the outer surface of the cover.
The tear seam may also be produced after molding the air bag cover skin, i.e., in a "post-molding operation", by scoring the skin with a knife, hot tool, laser, or other instrument to produce a thinned section in the pattern of a tear seam. However, such post molding operations are costly and must be carefully controlled.
Still another way to produce a weakened tear seam is to form the tear seam region from a weaker material than the rest of the air bag cover skin is formed from. Methods of this type are disclosed in U.S. Pat. Nos. 5,530,057 and 5,567,375, both assigned to the assignee of the present invention. Each of these patents disclose a casting process for forming an integral tear seam from plastic powder. Unlike the plastic powder used to form the rest of the cover skin, the plastic powder cast to form the tear seam region includes a "filler" material such as glass or carbonate. The resulting mixture has significantly less tensile strength than the plastic powder cast to form the rest of the cover skin. However, this method adds steps and cost to the manufacturing process.
It is desirable, when constructing an air bag door cover, to use a cover skin material that can withstand the harmful effects of extended exposure to solar radiation in the ultraviolet portion of the electromagnetic spectrum. Polymeric materials commonly used in forming air bag cover skins are subject to a phenomenon known as photodegradation caused by exposure to UV radiation and oxygen. Photodegradation comprises two distinct chemical processes that together result in chain scission and/or crosslinking in polymer molecules, i.e., the breakage of polymer bonds. In the first of the two processes, photolysis, a polymer absorbs UV radiation which breaks molecular bonds within the polymer forming free radicals. In the second process, autoxidation, the free radicals interact with oxygen to form peroxy radicals.
When polymeric materials are "weathered" in this manner, a strong change in morphology results which affects the mechanical behavior of the polymer. For example, the density and crystallinity of the polymer typically increase while its molecular weight and surface roughness decrease. This results in polymer embrittlement which correspondingly reduces the tensile strength and/or the percent elongation measured "at the break," i.e. at the point when the material breaks under a tensile load.
Photodegradation can be controlled or eliminated by including UV stabilizers or "inhibitors" which prevent these chemical reactions from occurring. UV promoters are also available that can actually accelerate these chemical reactions. Both UV inhibitors and promoters may be formulated to act only within a desired wavelength range.
UV inhibitors are generally formulated to inhibit UV light in the wavelength range from 320 to 390 nanometers--the so-called "UV-A" range where the ozone layer absorbs very little solar UV radiation. UV inhibitors may also be formulated to cover the wavelength range from 280 to 320 nanometers--the "UV-B" range where most, but not all solar UV radiation is absorbed in the ozone layer. Within the wavelength range from 150 to 280 nanometers, i.e., the "UV-C" range, the ozone layer absorbs practically all solar UV radiation with peak ozone absorption occurring at approximately the 250 nanometer wavelength.
What is needed is a simple, cost effective means of producing an air bag cover having tear seam that is both cost effective to produce and visually undetectable on the outer class A surface of an air bag cover skin.